I think a lower Kelvin 250w bulb may be overkill at 18"... or at least not NEEDED. I'd never do a single mh over 36" at any rate so I'd do 2 x 150w 14k or 2 x 250w 20k. I think a 36" T5 on either side of the mh's would be nice too so you can view the tank longer than the period of your mh's.
8 T5 bulbs on their own would work very well too.
250w gear is much more versatile as it can be carried over to your next upgrade...
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